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Shell-Induced Ostwald Ripening: Simultaneous Structure, Composition, and Morphology Transformations during the Creation of Hollow Iron Oxide Nanocapsules Lei Yu,† Ruixin Han,† Xiahan Sang,‡ Jue Liu,§ Melonie P. Thomas,† Bethany M. Hudak,⊥ Amita Patel,† Katharine Page,§ and Beth S. Guiton*,† †
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States Center for Nanophase Materials Sciences, §Neutron Scattering Division, and ⊥Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
S Supporting Information *
ABSTRACT: The creation of nanomaterials requires simultaneous control of not only crystalline structure and composition but also crystal shape and size, or morphology, which can pose a significant synthetic challenge. Approaches to address this challenge include creating nanocrystals whose morphologies echo their underlying crystal structures, such as the growth of platelets of twodimensional layered crystal structures, or conversely attempting to decouple the morphology from structure by converting a structure or composition after first creating crystals with a desired morphology. A particularly elegant example of this latter approach involves the topotactic conversion of a nanoparticle from one structure and composition to another, since the orientation relationship between the initial and final product allows the crystallinity and orientation to be maintained throughout the process. Here we report a mechanism for creating hollow nanostructures, illustrated via the decomposition of β-FeOOH nanorods to nanocapsules of α-Fe2O3, γ-Fe2O3, Fe3O4, and FeO, depending on the reaction conditions, while retaining single-crystallinity and the outer nanorod morphology. Using in situ TEM, we demonstrate that the nanostructured morphology of the starting material allows kinetic trapping of metastable phases with a topotactic relationship to the final thermodynamically stable phase. KEYWORDS: hollow nanorods, in situ TEM, iron oxide, Ostwald ripening, phase transformation
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well as the morphology, during the synthesis process. Generally, hollow nanomaterial syntheses can be categorized into template and template-free syntheses. Templated syntheses may utilize hard templates such as polystyrene,18 silica,19 carbon,20 and metal oxide21 or soft templates including gas bubbles22 and emulsion micelles.23 Due to the complexity of the process, however, template syntheses may be expensive and timeconsuming24,25 and often yield poorly crystalline materials. Template-free synthesis methods, such as galvanic erosion,26−29 the Kirkendall effect,30,31 and inside-out Ostwald ripening,32−34 come with advantages such as simple synthetic procedures, high reproducibility, low production costs, and the potential of
ollow nanomaterials have been studied for decades, in large part for their potential applications for catalysis,1−3 energy storage,4,5 sensors,6 and drug 7 delivery, which rely on the unique characteristics of a hollow morphology8 such as the high surface area to provide more active sites, the thin shell to reduce mass transport path, and the hollow interior to relieve structural strain with improved stability. Hollow iron oxide nanomaterials (such as α-Fe2O3) are of particular interest, due to their high theoretical lithium ion storage capacity, low toxicity, and low cost.9 For example, iron oxide nanotubes have been shown to perform well as anode materials in lithium ion batteries,10,11 supercapacitors,12 and catalysts13,14 and also display interesting magnetic properties.14−17 In order to fully exploit the potential of these hollow iron oxide nanomaterials, it is critical to understand how to control the crystal structure (for the many different iron oxide phases) as © XXXX American Chemical Society
Received: April 19, 2018 Accepted: August 21, 2018
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DOI: 10.1021/acsnano.8b02946 ACS Nano XXXX, XXX, XXX−XXX
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Cite This: ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano scaling up the synthesis for large quantity production. One promising template-free synthesis method for hollow iron oxide nanomaterials controls the phase transformation of iron oxidehydroxide Akaganéite, β-FeOOH, to form porous or hollow structures16,17,35−37 either forming α-Fe2O3 in air37−40 or the spinel phase (γ-Fe2O3 or Fe3O4) in high vacuum,35,36,39,41 possibly induced by the formation of voids or pores from decomposition of β-FeOOH.35,37 This method has proven economic and efficient, yet the mechanism governing the formation of these hollow structures has not been fully understood. In situ experiments such as in situ transmission electron microscopy (TEM) and in situ transmission X-ray microscopy have been exploited with great effect to reveal hollowing mechanisms by providing real-time information on morphology, composition, and crystal structure during reaction,28,29,33,34,42,43 leading to direct observation and interpretation of growth mechanisms. Recent advances in TEM have enabled spatial resolution of
